1. Field of the Invention
This invention concerns the continuous casting of metals, especially steel.
2. Description of the Related Art
The continuous casting operation consists schematically, as is known, in pouring a molten metal into an ingot mould mainly consisting of a tubular element without a bottom, defining a passageway for the cast metal. The walls of the mould, made of copper or, more generally of a copper alloy, are energetically cooled by circulating water. The product already solidified externally over a thickness of several centimeters is continuously extracted from the mould. The solidification then progresses towards the centre of the product and is completed during the descent of the product downstream of the ingot mould in the so-called "secondary cooling" zone under the effect of water spray lines. The product obtained, the bloom, billets or slab, is next cut to length then rolled before being shipped to the customer or transformed in situ into bars, wires, sections, plates, sheets, etc..
Surface defects or defects under the surface of the products obtained by the continuous casting of steel often lead to scrapping as they are not well tolerated by the rolling operation or are even amplified by this operation which may go as far as intolerably degrading the metallurgical quality of the rolled products.
During casting, the molten metal, fed into the ingot mould via a nozzle, forms a solid film when it comes into contact with the cooled walls of the ingot mould. This film is driven downwards during the extraction of the product by jerky movements punctuated by the vertical oscillations of the ingot mould and, simultaneously, its thickness increases due to continued heat extraction via the walls of the ingot mould. Therefore, a new film of solid metal is continuously created at the level of the free surface of the metal in the ingot mould, this film solidifying over the complete perimeter of the inner wall of the ingot mould and thus comprising a solid ring liable to contract due to the cooling to which it is submitted during its descent in the ingot mould.
The contraction of the ring is augmented as heat extraction increases and by the natural tendency of the cast metal to contract during cooling, for example by change of solid phase at end of solidification as is the case especially for the 0.1% carbon steel or stainless steel AISI 304 grades.
This peripheral contraction tends to separate the solidified skin from the wall of the ingot mould and therefore leads to a reduction in the heat exchange as the contact of the said skin with the cold walls is degraded. This separation is generally unequal over the perimeter of the solidified skin and is a source of surface defects in the product finally obtained.
To avoid or limit these defects, a specific technique not yet industrialised, known as vertical continuous casting, consists in placing a hot-top element made of a heat-insulating refractory material above the cooled metal walls of the ingot mould and in maintaining, during casting, the free surface of the metal bath at the level of the said hot-top element (French patent No. 2000365). Thus, the molten metal does not solidify in contact with the hot-top element, the first solidified skin starting to form only from the upper edges of the cooled metal wall. As these edges are located at a sufficient distance below the disturbed zone near to the free surface, the creation and the growth of the solid skin is achieved continuously always at same level in the ingot mould, in a calm environment from a hydrodynamic viewpoint, in the region where the ferrostatic pressure exerted by the weight of the liquid metal located above opposes the tendency of the first solidified skin to separate from the cold wall of the ingot mould.
In this technique, an improvement, known by document EP-A-O 620 062, consists in injecting, into the ingot mould, at the level of the said hot-top element and at least just at the interface between the said hot-top element and the cooled metal walls, an inert gas under pressure. This gas injection, made via a thin annular slot made between the said walls and the hot-top element, forms jets perpendicular to the walls and directed towards the liquid metal which shear any solidified skins which may have formed in contact with the refractory hot-top element in order to ensure that solidification effectively starts exactly at the upper edge of the cooled walls.
Although this technique in principle enables the appearance of certain surface defects on the finished product to be reduced, it does not however solve the problems concerning the adaptation of the casting process to the various families of steel grades which can be continuously cast to take into account the specificities of each of the grades concerning their thermomechanical behaviour during solidification.